The present invention relates to color imaging. More particularly, the present invention relates to mapping colors between color imaging systems.
Color reproduction processes typically involve using color imaging systems to produce colors on various media. These color imaging systems may be used to duplicate a color image from one medium to another medium, e.g., from one printed copy to another or from a display screen to a printed copy. Color reproduction processes are used in various application environments, for example, color proofing applications.
Some color reproduction processes use approaches known as color management systems (CMSs) to characterize various color imaging systems and to transform color data between the color imaging systems. Characterizing color imaging systems typically involves calculating color response functions using color coordinate systems known as color spaces. One commonly-used color space is Commission Internationale de l""Éclairage L*a*b* (CIELAB) space. CMSs attempt to reproduce an original color image on a color imaging system so as to preserve the appearance of colors between the original and the reproduction within the limitations of the color imaging system of the reproduction process.
Various CMS approaches have been proposed to achieve accurate color reproduction. Many of these approaches involve producing color samples using an output or display device and measuring the color values of the samples using an input device. Such approaches correlate the output colors with the measured color values. This correlation is performed using, for example, forward and reverse transforms between device-specific color spaces and a device-independent color space. These transformation techniques are often supplemented by interpolation between entries in a multidimensional lookup table. These techniques exhibit inaccurate color conversion between similar devices, potentially resulting in undesirable contamination of colors. Furthermore, accurate color conversion of dark colors has often been particularly difficult because of inadequate processing of black channel data in many applications.
CMSs often perform gamut mapping to correlate the range or gamut of colors that can be realized by a device with regions of a color space. Because many devices are incapable of realizing the complete range of colors in a color space, gamut mapping typically involves compressing or scaling regions of the color space. The device can then approximate colors outside its gamut using the compressed regions of the color space. For many CMSs, gamut mapping is potentially inconsistent under certain circumstances, such as when using profiles generated by software from different vendors. In addition, many CMSs exhibit inconsistencies when performing forward and reverse transformations between imaging systems. For example, color shifting often occurs with repeated forward and reverse transformations.
Many CMS techniques exhibit other limitations in addition to the lack of accuracy in converting colors. For example, many CMS techniques are relatively inflexible with respect to changes in illumination and observer conditions, gamut mapping, and choice of color space. Certain techniques lack forward compatibility with future color standards.
According to one embodiment, the present invention is directed to a color mapping method for use in transforming colors between color imaging systems. The color mapping method includes using forward transformation profiles characterizing the color imaging systems to generate respective sets of device-independent color values for the color imaging systems. Color conversions are calculated by iteratively reducing differences between the sets of device-independent color values. This difference reduction is also optionally performed on black channel information to obtain a mapping of black channels between the color imaging systems. A color map describing a relationship between the color imaging systems is constructed by using the predicted color conversions. This method may be performed by a color mapping arrangement or a computer-executable program stored on a data storage medium.
According to another embodiment of the present invention, color mapping between imaging systems is accomplished by a conversion using profiles that characterize the color imaging systems to generate respective sets of device-independent color values for the source color imaging system and converting to the device-dependent data values of the destination color imaging system by performing a color conversion using the profiles. The device-independent color values have the same dimensionality as the corresponding color imaging systems. The color conversion can be used to improve its own accuracy relative to a quality threshold. The color conversion is used to define a color map for transforming colors between the color imaging systems.
Another embodiment of the present invention is directed to a color mapping arrangement for use in transforming colors between imaging systems. A computer arrangement uses forward transformation profiles that characterize the color imaging systems to generate respective sets of device-independent color values for the color imaging systems. The computer arrangement also calculates color conversions by iteratively reducing differences between the sets of device-independent color values. The computer arrangement uses the color conversions to construct a color map describing a relationship between the color imaging systems using the color conversions. A memory stores the color map.
The above summary of the invention is not intended to describe each disclosed embodiment of the present invention. This is the purpose of the figures and of the detailed description that follows.